AIM is a minisatellite mission
within NASA's SMEX (Small Explorer) program designed to provide
frequent, low-cost access to space for a variety of missions (the AIM
mission was selected in July 2002 with final approval in May 2004). The
objective of AIM is to study the causes of Earth's highest-altitude
clouds, which occur on the very edge of space. These clouds, referred
to as PMCs (Polar Mesospheric Clouds), form in the coldest part of the
atmosphere, about 50-90 km above the polar regions, every summer. 1)

PMCs are of special interest as they
are sensitive to both global change and solar/terrestrial influences
(study of the coupling between the heliosphere and the Earth's
atmosphere). Recorded sightings of these silvery-blue, noctilucent or
”night-shining” clouds (NLCs) were first reported in
1885 at high latitudes. They have been increasing in frequency and
extending to lower latitudes over the past four decades. They are
called ”night shining” clouds by observers on the ground
because their high altitude allows them to continue reflecting sunlight
after the sun has set below the horizon.

The AIM mission will observe PMCs in
their thermal, chemical and dynamic environment in which they form in
order to determine the connection between PMCs and the meteorology.
Specific parameters of the polar mesosphere to be measured are: PMC
abundances, spatial distribution, particle size distributions, gravity
wave activity, cosmic dust influx to the atmosphere and precise,
vertical profile measurements of temperature, H2O, OH, CH4, O3, CO2,
NO, and aerosols. The results from this mission will provide the basis
for study of long-term variability in the mesospheric climate.

AIM is a NASA PI (Principal
Investigator) mission, lead by James M. Russell III of Hampton
University (HU), Hampton, VA. The AIM team, led by HU, is made up of
members from varies partner organizations (universities and
institutions). In this setup, Hampton University is the prime
contractor to NASA and manages all programmatic aspects of the project.
LASP (Laboratory for Atmospheric and Space Physics) of the University
of Colorado at Boulder is a major subcontractor to AIM, providing two
instruments and the functions of mission operations and data
acquisition. AIM data will be analyzed and prepared for public
archiving by Hampton University with the assistance of GATS (Gordley
&Associates Technical Software) Inc. of Newport News, VA. Further
partners in AIM are: UAF (University of Alaska, Fairbanks), USU/SDL
(Utah State University / Space Dynamics Laboratory), GMU (George Mason
University), and BAS (British Arctic Survey). 2)3)4)

OSC (Orbital Sciences Corporation)
of Dulles, VA, is the prime contractor to CAS/HU (Center for
Atmospheric Sciences/Hampton University) for the spacecraft and payload
integration. The AIM mission employs the LeoStar-2 bus of OSC, a 3-axis
stabilized zero momentum platform. The spacecraft structure (hexagonal
bus) has a diameter of 1.09 m and a length of 1.4 m. S/C power = 335 W
(orbital average) using a fixed GaAs solar array; spacecraft mass of ~
200 kg, design life of at least 2 years. 5)6)

The spacecraft uses an aluminum
honeycomb bus structure, with a deployed solar array wing canted at
50º. The solar array uses high efficiency solar cells on composite
substrates, and the full array is assembled from 6 solar panel
sections, and wrapped around the spacecraft (cells facing out) during
launch. The panels are sequentially deployed into a flat panel and then
canted away from the spacecraft body. The array provides 300 W of
average power. The instruments SOFIE and CIPS are mounted to the nadir
panel of the hexagonal structure, and CDE is mounted to the zenith
panel. Instrument electronics for SOFIE are housed within the
spacecraft body.

Figure 3: The cylindrical bus structure of AIM (image credit: NASA)

Attitude control is accomplished
using 3 reaction wheels and 3 torque rods. The C&DH system employs
a RAD 6000 computer (BAE). The ACS system uses a star tracker, gyro,
magnetometer and sun sensors.

The spacecraft is inertially pointed
during SOFIE observations, and nadir oriented with pitch and roll
offsets to obtain the common volume CIPS observations. The spacecraft
performs a subsolar yaw maneuver to keep the arrays sunlit over the
daylight side of the orbit.

Legend to Figure 5:
There are six separate solar array panels, which are integrated into a
single deployed wing. In this orientation the nadir deck is pointed up
towards the ceiling, and the SOFIE and CIPS instruments are visible in
the photo. The photograph was taken in the clean room at the Orbital
Sciences, Dulles, Va. facility.

Launch: An air launch of AIM
on the Pegasus-XL launch vehicle of OSC took place on April 25, 2007
(air launch from an L1011 aircraft near the launch site: VAFB, CA,
USA). 7)

RF communications: An S-band link is
chosen via NASA's space/ground network and TDRS (Tracking and Data
Relay Satellite) system. Communication is done through two helix
antennas using an L-3 CXS-600B transceiver with S-band uplink and two
downlink transmit capabilities for compatibility with the 2 Msample/s
transmit rate to the ground network (GN), and the 2 ksample/s rate to
TDRS.

• May 28, 2020: Ice-blue clouds
are drifting high above the Arctic, which means the Northern
Hemisphere’s noctilucent cloud season is here. NASA's AIM
spacecraft first spotted wisps of noctilucent, or night-shining, clouds
over the Arctic on May 17. In the week that followed, the ghost-like
wisps grew into a blur, quickly filling more of the Arctic sky. This is
the second-earliest start of the northern season yet observed, and the
season is expected to run through mid-August. 8)

Figure 7: These animated images
show AIM’s observations from the first week of the Arctic
noctilucent cloud season, which began on May 17, 2020. The colors
— from dark blue to light blue and bright white — indicate
the clouds’ albedo, which refers to the amount of light that a
surface reflects compared to the total sunlight that falls upon it.
Things that have a high albedo are bright and reflect a lot of light.
Things that don’t reflect much light have a low albedo; they are
dark (image credit: NASA/HU/VT/CU-LASP/AIM/Joy Ng)

- The seasonal clouds hover high
above the ground, about 50 miles overhead in a layer of the atmosphere
called the mesosphere. Most meteors burn up when they reach the
mesosphere; there are enough gases there to slough plummeting meteors
into nothing more than dust and smoke. Noctilucent clouds form when
water molecules congregate around the fine dust and freeze, forming ice
crystals. The icy clouds, reflecting sunlight, shine bright blue and
white. They first appear in summer — around mid-May in the
Northern Hemisphere and mid-November in the Southern — when the
mesosphere is most humid, with the season’s heat lofting moisture
up to the sky.

- “Every year, twice a year,
the start of the season is a big event for us,” said Jim Russell,
AIM principal investigator at Hampton University in Virginia.
“The reason we’re excited is we’re trying to find out
what the causes of the season’s starting are and what does it
really mean with regard to the larger picture in the atmosphere.”

- Also known as polar mesospheric
clouds (because they tend to huddle around Earth’s poles), these
clouds help scientists better understand the mesosphere and how
it’s connected to the rest of the atmosphere, weather and
climate.

- Scientists are eager to see what
this Arctic season brings. For the most part, the brilliant clouds
usually cling to the polar regions. But sometimes, they stray south.
Last year, they were spotted as far south as southern California and
Oklahoma — lower latitudes than have ever been seen before,
Russell said. The new season is another chance to better understand the
fleeting clouds and their possible migration south. Some evidence
indicates this could be the result of changing atmospheric conditions.

- “With every year, we get new data to help us put together a picture of the atmosphere,” Russell said.

- Launched in 2007, AIM is a
NASA-funded mission managed by NASA’s Goddard Space Flight Center
in Greenbelt, Maryland. The mission is led by the AIM principal
investigator from the Center for Atmospheric Sciences at Hampton University.

• February 11, 2020: The AIM
mission is operating and about to complete its 13th year in orbit on 25
April. The two AIM instruments are operating nominally and collecting
excellent data (Ref. 10).

• January 2020: A paper has
been published. The authors have updated long‐term trends in mesopause
temperature, airglow emission intensities and noctilucent clouds (NLC)
based on ground‐based observations conducted in the Moscow region
(Russia). Trends in mesopause temperature and airglow emissions have
been derived for the period 2000‐2018 (19 years), and long‐term trends
in NLC characteristics have been obtained for 1968‐2018 (51 years).
Trends in airglow emissions have been estimated separately for winter
and summer seasons. 9)

• January 9, 2019: AIM is fully
operational and collecting scientific data using nadir measurements of
Rayleigh backscattered sunlight made by the CIPS (Cloud Imaging and
Particle Size)high spatial resolution
camera and limb extinction using the SOFIE(Solar Occultation For Ice
Experiment) solar occultation approach. It is routinely making
measurements of Polar Mesospheric Clouds during the cloud season by
both CIPS and SOFIE, and SOFIE profiles at all times of temperature, O3, H2O, CH4,
NO and limb extinction at eight wavelengths ranging from 0.87 to 4.6
µm. Outside the PMC region, CIPS provides high spatial resolution
measurements of gravity waves near 50 km altitude over a broad latitude
range extending ranging from high northern to high southern latitudes
(Ref. 10).

• January 30, 2018: After more
then 10 years on orbit, the AIM mission is fully operational with
planned operations going through September 2023. 10)

• January 7, 2018: The sky over
Antarctica is now glowing electric blue with noctilucent, or
night-shining, clouds. That’s according to recent images from
NASA’s AIM spacecraft (Aeronomy of Ice in the Mesosphere), which
monitors these clouds for the whole Earth. The season for night-shining
clouds in the Southern Hemisphere is November to April, so they are
right on schedule. These are ice clouds, and Earth’s highest
clouds, located some 50 miles (80 km) above the ground in a layer of
the atmosphere called the mesosphere. The clouds – made of ice
crystals – are seeded by fine debris from disintegrating meteors.
11)12)

- The season for
noctilucent clouds in the north is May to September. In both
hemispheres, they happen when it’s summertime, when water vapor
wafts up into the high atmosphere, providing the moisture needed to
form these spectacular ice clouds at the edge of space. - As for the
electric-blue glow, it comes from sunlight shining through the high
clouds.

- Cora Randall, a member of the AIM
science team at the University of Colorado’s Laboratory for
Atmospheric and Space Physics, said: ”The current season began on
November 19. Compared to previous years of AIM data, this season seems
to be fairly average, but of course one never knows what surprises lie
ahead, particularly since the southern hemisphere seasons are so
variable.”

- If you were in Antarctica now,
would you see these clouds shining overhead? Not likely, since
there’s 24-hour daylight shining on that part of the globe now.
But we’re past the December solstice, meaning that summer is
waning in the Southern Hemisphere. People outside the Antarctic, at
relatively high Southern Hemisphere latitudes, might be able to glimpse
the clouds, especially as their sunsets come earlier and night
lengthens on that part of the globe.

- We frequently see images of
noctilucent clouds, taken from the ground, during the northern summer.
Our friends at high northern latitudes – typically from northern
Europe and Scandinavia – capture them.

• November 9, 2016: NASA
honored the AIM Flight Operations Team (FOT) with a Group Achievement
Award for its exceptional engineering and innovative achievement
enabling the AIM mission to continue operations without command uplink.
13)

- The AIM spacecraft has now gone
1193 days without command uplink capability, lost early in this
single-string, low-cost mission due to a defect in the command receiver
exacerbated by spacecraft charging and radiation effects. The
extraordinary innovation and dedication of the AIM FOT clearly saved
this mission, enabling significant scientific advancements in
understanding the coldest region on Earth. Their efforts have pushed
the limits in spacecraft automation beyond anything accomplished
before. They enabled science operations to continue despite an anomaly
that could have ended the AIM mission prematurely, and significantly
reduced the cost of operating the mission during the extended mission
phase.

- Ten days into the mission, AIM
began having difficulty attaining lock on the uplink subcarrier. Since
then, the AIM spacecraft experienced varying periods, from hours to
weeks, between contacts with successful command uplink. The first
extended outage of approximately four days occurred a couple of weeks
after the problem surfaced. This drove the FOT to examine new ways of
operating the spacecraft. The FOT made massive software changes while
they could, to allow the spacecraft to operate autonomously for long
periods without command uplink. The initial efforts focused on
providing a system that was robust against extended command outages
without making significant changes to the risk posture of the program.

- Then, the priority shifted to
developing and testing groundbreaking techniques for the command and
control of a deaf satellite. The first efforts focused on producing an
expedited instrument commissioning sequence using stored commands and
developing a stored command sequence to execute in the event of another
extended outage. Next, the FOT increased the onboard command storage
capability, improved the on-board orbit knowledge and provided for an
autonomous downlink of recorded data when AIM passed over a scheduled
ground station. Then the FOT proceeded to fully automate the spacecraft
to handle science observation sequences and to perform autonomous orbit
maneuvers using the onboard telemetry monitors that are nominally used
for onboard fault detection and correction sequences. This provided the
AIM mission with a means for continuing science observations in the
event of an extended command outage.

- Having achieved a level of
autonomy that would meet mission requirements, the FOT worked on
improving the science data quality and the robustness of the system.
This involved incorporating the ground based mission-planning software
into flight software. In parallel, the FOT developed a process to
modulate the RF signal through the TDRSS link in ways that are
observable to the flight software (a.k.a. Morse code commanding),
without requiring the receiver to lock onto the subcarrier. This allows
the operations team to trigger pre-loaded stored command sequences to
perform emergency recovery operations. The capability to build custom
command sequences is enabling new science as the AIM orbit evolves
during extended mission operations.

- The AIM spacecraft will be
reconfigured to perform science during upcoming years of "full-sun"
condition when the autonomous state vector routine and the on-board
mission planning software will no longer function due to lack of
sunrises and sunsets. These innovative modifications continue to enable
robust ongoing spacecraft operations with no loss of science data
quality or quantity. The FOT implemented extensive, opportunistic and
highly creative operational changes to both the spacecraft and science
instruments, resulting in the ongoing return of over 98% of the science
data on a continuous basis after accomplishing 100% of the science
return for the mission. Their accomplishment has enormous potential for
the future conscious implementation of spacecraft autonomy, potentially
requiring fewer operators and lower cost to NASA. This was all
accomplished with no change of funding from NASA and resurrected a
mission that could have been a total loss extending it into years of
spectacular science results.

- As of February 2017, the AIM orbit
plane is nearly perpendicular to the Earth - sun vector. As a result
the sun does not rise or set as viewed from the satellite, and SOFIE
measurements are not possible. This will change by early October, 2017,
when SOFIE will resume measurements. SOFIE science and housekeeping
parameters all indicate a stable and healthy instrument. SOFIE V1.3
data are available online through February 2017.

- Siskind et al. 15)
used SOFIE and MLS satellite data to categorized the inter-annual
variability of winter and springtime upper stratospheric methane (CH4).
They showed the effects of this variability on the chemistry of the
upper stratosphere throughout the following summer. Years with strong
wintertime mesospheric descent followed by dynamically quiet springs,
such as 2009, lead to the lowest summertime CH4.

- Years with relatively weak
wintertime descent, but strong springtime planetary wave activity, such
as 2011, have the highest summertime CH4. By sampling Aura
MLS to the SOFIE measurement locations, it was demonstrated that
summertime upper stratospheric ClO almost perfectly anti-correlates
with the CH4 (Figure 9). This is consistent with the reaction of atomic chlorine with CH4
to form the reservoir species, hydrochloric acid (HCl). The summertime
ClO for years with strong uninterrupted mesospheric descent is about
50% greater than in years with strong horizontal transport and mixing
of high CH4 air from lower latitudes. Small, but persistent
effects on ozone are also seen such that between 1 and 2 hPa, ozone is
about 4–5% higher in summer for the years with the highest CH4 relative to the lowest. This is consistent with the role of the chlorine catalytic cycle on ozone.

- These
dependencies may offer a means to monitor dynamical effects on the
high-latitude upper stratosphere using summertime ClO measurements as a
proxy. Additionally, these chlorine-controlled ozone decreases, which
are seen to maximize after years with strong uninterrupted wintertime
descent, represent a new mechanism by which mesospheric descent can
affect polar ozone. Finally, given that the effects on ozone appear to
persist much of the rest of the year, the consideration of
winter/spring dynamical variability may also be relevant in studies of
ozone trends.

Figure 9:
The color contours on the left are zonal mean WACCM/NOGAPS difference
fields for August 2009 minus August 2008 for ClO (top) and CH4
(bottom). The vertical dashed white line is the mean latitude of the
SOFIE occultations for August. On the right, a vertical profile of the
model difference at the SOFIE occultation latitude (solid line with
plus symbols) is compared with MLS ClO and SOFIE CH4 (data are
dotted/dashed curves with stars). Note that x axes for the right panels
are reversed from one another since the ClO change is positive, while
the CH4 change is negative (image credit: NRL, Hampton University,
Virginia Tech, University of Colorado)

• December 02, 2016: Strictly
speaking, noctilucent or “night shining” clouds don’t
glow in the dark the way the bioluminescent algae or fireflies do. Also
called polar mesospheric clouds, they float high enough in the
atmosphere to capture a little bit of stray sunlight, even after the
Sun has fallen below the horizon. In this way, they have more in common
with an actor being illuminated by a spotlight in front of a dark
curtain. 16)17)

- Noctilucent clouds commonly occur
at high latitudes in the late spring and early summer, before 24-hour
daylight sets in. They form around both the North and South Pole,
roughly 50 to 85 km above the Earth’s surface. NASA's AIM
captured images of the clouds over Antarctica on November 29, 2016, a
few weeks after they first appeared on November 17. The mosaic of
Figure 10 was created using satellite data
from several passes over the Antarctic. The AIM instrument measures
cloud albedo—the amount of light reflected back to space. Lighter
areas correspond to more brightly lit clouds, while areas with no data
appear in black.

- The AIM mission first observed
noctilucent clouds over the Arctic in 2007, and the clouds seem to be
appearing earlier and at lower latitudes. They have been observed as
far south as Utah and Colorado. Research has also shown that they have
been getting brighter.

Figure 10:
Data from the AIM spacecraft shows the sky over Antarctica is glowing
electric blue due to the start of noctilucent, or night-shining, cloud
season in the Southern Hemisphere. This data was collected from Nov.
17-28, 2016 (image credit: NASA/HU/VT/CU-LASP/AIM/Joy Ng, producer)

• September 26, 2016: The AIM spacecraft and its instruments (SOFIE and CIPS) are working very well, in their 9th
year on orbit. The project is dealing with a temporary issue brought on
by orbit precession which has caused the loss of data for about a
month. However, the problem is understood and steps are underway to fix
the problem. The full collection mode should be reached in about two
weeks. 18)

• March 8, 2016: 19)
The SOFIE instrument continues to operate normally. Software
modifications to accommodate the changing orbit and updated instrument
operations are ongoing. The changes are confined to level 1 and are
addressing times when the SOFIE attenuator balance procedure approached
exoatmospheric heights, and the new operation sequence (post September
2015) which has longer events and no balance procedure. SOFIE
observations of temperature versus altitude have been used to
characterize GWs (Gravity Waves). 20)21)
The technique described by Thurairajah et al. (Ref. 21)
was used to derive vertical profiles of GW amplitude and PE (Potential
Energy) from SOFIE results for May 2007 through December 2014.

• July 10, 2015: The AIM
mission is approved by NASA to continue planning against the current
budget guidelines. Any changes to the guidelines will be handled
through the budget formulation process. The AIM mission will be invited
to the 2017 Heliophysics Senior Review. 22)

- June 2015: The 2015 Heliophysics
Senior Review panel undertook a review of 15 missions currently in
operation in April 2015. The panel found that all the missions continue
to produce science that is highly valuable to the scientific community
and that they are an excellent investment by the public that funds
them. 23)

- AIM demonstrated that water vapor
injections into the upper mesosphere and lower thermosphere (MLT) from
space traffic potentially affect polar mesospheric cloud properties and
compensate for stronger water vapor photolysis during increased solar
activity. Observational evidence from AIM and TIMED signal decreases in
MLT composition and temperature during the last decade, indicating
possible anthropogenic cooling effects due to rising concentrations of
the greenhouse gas CO2. Insights like these into the sources
of natural and human induced changes in the Earth system address a
primary science theme of the Earth Science Division.

• June 19, 2015: In the late
spring and summer, unusual clouds form high in the atmosphere above the
polar regions of the world. As the lower atmosphere warms, the upper
atmosphere gets cooler, and ice crystals form on meteor dust and other
particles high in the sky. The result is noctilucent or
“night-shining” clouds (NLCs)—electric blue wisps
that grow on the edge of space. 24)

Legend to Figure 12:
This image is a composite of several satellite passes over the Arctic,
and the clouds appear in various shades of light blue to white,
depending on the density of the ice particles. The instrument measures
albedo—how much light is reflected back to space by the
high-altitude clouds.

• Feb. 27, 2015: The AIM mission is in its 8th
year on orbit and is operating very well. The CIPS (Cloud Imager and
Particle Size) and SOFIE (Solar Occultation For Ice Experiment)
instruments are working without flaw. The dust instrument, CDE (Cosmic
Dust Experiment), was turned off about 4 years ago. So everything is
working very well (Ref. 29).

• October 31,
2014: All systems on AIM are functioning nominally. Also, in order to
mitigate the effects of the solar eclipse which occurred on October
23rd, AIM was transitioned to its backup attitude control mode
(TMON/RTS Control) prior to the eclipse, and transitioned back to OOMP
(the normal control mode) on the evening of October 24, 2014. 25)

- The CIPS instrument continues to
perform well, with no health issues. The project is gearing up for the
start of the Southern Hemisphere (SH) season, which should begin later
in November or early December. It was previously suggested that
interhemispheric teleconnections triggered by planetary wave activity
in the SH winter stratosphere led to a rapid decrease in PMCs (Polar
Mesospheric Clouds) in the Northern Hemisphere (NH) that began about 40
DFS (Days From Solstice). The PMCs recovered significantly after
reaching a minimum near DFS 45, peaking near DFS 53; the season ended
shortly thereafter. See the first figure below, which shows the daily
PMC frequency at 80°N latitude for all NH PMC seasons observed by
AIM; the red curve shows 2014.

- The SOFIE instrument continues to
operate nominally, and is collecting high quality data on the state of
the middle atmosphere. Observations of the Northern Hemisphere (NH)
2014 season are complete and relevant data files and figures are
available on the SOFIE and AIM web pages.

• April 2014: New data from
NASA's AIM spacecraft have revealed "teleconnections" in Earth's
atmosphere that stretch all the way from the North Pole to the South
Pole and back again, linking weather and climate more closely than
simple geography would suggest. 26)

- The AIM science team at HU
(Hampton University) and at LASP (Laboratory for Atmospheric and Space
Physics), University of Colorado, Boulder, CO, found that the winter
air temperature in Indianapolis, Indiana (or any other city in the
USA), is well correlated with the frequency of NLCs (Noctilucent
Clouds) over Antarctica. NLCs are Earth's highest clouds. They form at
the edge of space 83 km above our planet's polar regions in a layer of
the atmosphere called the mesosphere. Seeded by "meteor smoke," NLCs
are made of tiny ice crystals that glow electric blue when sunlight
lances through their cloud-tops.

- While in the
first part of the AIM mission, the project's attention was focused on a
narrow layer of the atmosphere where NLCs form. Now the project team is
finding out this layer manifests evidence of long-distance connections
in the atmosphere far from the NLCs themselves.

- One of these teleconnections links
the Arctic stratosphere with the Antarctic mesosphere. Stratospheric
winds over the Arctic control circulation in the mesosphere. When
northern stratospheric winds slow down, a ripple effect around the
globe causes the southern mesosphere to become warmer and drier,
leading to fewer NLCs. When northern winds pick up again, the southern
mesosphere becomes colder and wetter, and the NLCs return.

- In January 2014, a time of year
when southern NLCs are usually abundant, the AIM spacecraft observed a
sudden and unexpected decline in the clouds. Interestingly, about two
weeks earlier, winds in the Arctic stratosphere were strongly
perturbed, leading to a distorted polar vortex.

- The AIM science team believes that
this triggered a ripple effect that led to a decline in NLCs half-way
around the world. This is the same polar vortex that made headlines
this winter (2013/2014) when parts of the USA experienced crippling
cold and ice.

Figure 14:
The 2013/2014 winter air temperature in Indianapolis is correlated with
the frequency of noctilucent clouds over Antarctica (image credit: AIM
Science Team)

Legend to Figure 14:
Changes in surface temperatures near Indianapolis, IN (blue, left and
bottom scales) are well correlated with changes in the Arctic
stratosphere (orange, right and bottom scales) and with changes in
noctilucent clouds (PMCs) at 77º S latutude two weeks later (red,
left and top scales).

• January 2014: The AIM spacecraft continues to perform nominally in its 7th year on orbit. The project received funding to operate through September 2018 (Ref. 29).

• June 2013: Every summer,
something strange and wonderful happens high above the north pole. Ice
crystals begin to cling to the smoky remains of meteors, forming
electric-blue clouds with tendrils that ripple hypnotically against the
sunset sky. This year, NLCs (Noctilucent Clouds) are getting an early
start. NASA's AIM spacecraft started seeing them on May 13. 27)

The early start is extra-puzzling
because of the solar cycle. Researchers have long known that NLCs tend
to peak during solar minimum and bottom-out during solar
maximum—a fairly strong anti-correlation.

• On April 25,
2013, the AIM spacecraft was 6 years on-orbit. AIM continues to operate
nominally. A lunar eclipse occurred on May 10 , 2013 but the coarse sun
sensors remained locked on the sun and therefore had no impact on the
spacecraft operations (Ref. 28).

- AIM is currently funded to operate
through September 2013. The project submitted a proposal to the NASA
Senior Review process for continued operations through 2018. That
proposal is in review with a decision expected in June, 2013. 29)

• August 2012: A key ingredient
of Earth's strangest clouds does not come from Earth. New data from
NASA's AIM spacecraft shows that "meteor smoke" is essential to the
formation of NLCs (Noctilucent Clouds). Using data from the SOFIE
(Solar Occultation for Ice Experiment), the project found that about 3%
of each ice crystal in a noctilucent cloud is of meteoritic origin. 30)

The inner solar system is littered
with meteoroids of all shapes and sizes — from asteroid-sized
chunks of rock to microscopic specks of dust. Every day Earth scoops up
tons of the material, mostly the small stuff. When meteoroids hit our
atmosphere and burn up, they leave behind a haze of tiny particles
suspended 70 km to 100 km above Earth's surface.

In the 19th century, NLCs were
confined to high latitudes—places like Canada and Scandinavia. In
recent times, however, they have been spotted as far south as Colorado,
Utah and Nebraska. The reason, James Russell (PI of AIM mission)
believes, is climate change. One of the greenhouse gases that has
become more abundant in Earth's atmosphere since the 19th century is
methane (CH4). It comes from landfills, natural gas and petroleum systems, agricultural activities, and coal mining.

When methane makes its way into the
upper atmosphere, it is oxidized by a complex series of reactions to
form water vapor. This extra water vapor is then available to grow ice
crystals for NLCs.

Figure 15: The graphic shows how
methane, a greenhouse gas, boosts the abundance of water at the top of
Earth's atmosphere. This water freezes around "meteor smoke" to form
icy noctilucent clouds (image credit: Hampton University, NASA)

• Status of July 20, 2012: All of the AIM spacecraft subsystems continue to perform well (Ref. 32).
During the last period of bitlock on May 23, the project loaded several
products to improve three areas of the spacecraft's performance.

Figure 16: Astronauts on board
the ISS took this picture of noctilucent clouds near the top of Earth's
atmosphere on July 13, 2012 Image credit: HU, NASA)

• The AIM spacecraft and its instruments are operating nominally (except for a command workaround) in 2012 - in its 5th year on orbit. The AIM mission has been extended by NASA through the end of FY12.

• The AIM spacecraft and its instruments are operating nominally in 2011 (Ref. 2).

• The AIM spacecraft and its instruments are operating nominally in 2010. The AIM mission has been extended by NASA through the end of FY12.32)33)

• For the first time scientists
have a comprehensive data set showing the formation and seasonal
variation of the clouds over both poles. The mission is providing high
quality data on cloud nucleus particle size, size variation with
altitude, particle shape and its altitude dependence, and other
characteristics that describe the onset and end of the PMC season. In
addition scientists are observing the interplay between particles,
water vapor and temperature variations, brightness variability over the
entire polar cap region, and space and time variability. 34)

All AIM spacecraft
systems have been functioning nominally since launch - except for the
command receiver. The receiver has had periods of intermittent command
signal rejection, but the AIM Flight Operations team has been able to
successfully work around these difficulties. 35)

• The autonomous operations
concept for AIM has evolved over its first year on orbit. On May 20,
2008, AIM has been selected for extended mission funding following the
2-year Explorer baseline mission. The extension from June 2009 through
September 2012 will allow tracking the evolution of mesospheric clouds
for an additional seven seasons and provide data to address key
outstanding questions including: 36)

- Are there variations in PMCs that can be explained by changes in solar irradiance and particle input?

- What changes in mesospheric properties are responsible for north/south differences in PMC features?

- What controls interannual variability in PMC season duration and latitudinal extent?

- What is the mechanism of teleconnection between winter temperatures and summer hemisphere PMCs?

- What is the global occurrence rate of gravity waves outside the PMC domain?

• Routine science data
processing started in February 2008. All data products are available to
the public via the internet at the main AIM web page (Ref. 35).

• As of December 2007, AIM has
provided the first global-scale view of the clouds over the entire 2007
Northern Hemisphere season with an unprecedented horizontal resolution
of 5 km x 5 km. 37)

• Full science operations began
on May 22, 2007. In June 2007, the AIM instruments captured the first
images of noctilucent clouds over the Arctic region. 38)

• The commissioning of the
spacecraft proceeded nominally through attaining normal pointing mode.
Nine days after launch, the satellite started to have problems locking
on the command uplink subcarrier modulation. This was the beginning of
the intermittent operation of the transceiver that has continued ever
since. Over the next couple of weeks many different uplink
configurations were tested to characterize the performance of the
receiver. 39)

The initial efforts focused on
providing a system that was robust against extended command outages
without making significant changes to the risk posture of the program.
Once that had been accomplished, the priority shifted to developing and
testing groundbreaking techniques for the command and control of a deaf
satellite. These enhancements are being used to ensure AIM continues to
collect great science data on the mysterious clouds that appear on the
edge of space.

Figure 18: One of the first
ground sightings of noctilucent clouds in the 2007 season over
Budapest, Hungary on June 15, 2007 (image credit: NASA)

Figure 19: Noctilucent clouds over the Arctic region as seen by the AIM instruments (image credit: NASA)

Sensor complement: (CIPS, CDE, SOFIE)

The sensor complementconsists of three instruments. Initially, the mission was planned with 4 instruments, but SHIMMER (Spatial Heterodyne Imager for Mesospheric Radicals) of NRL was deleted due to budgetary problems.

Instrument

Mass (kg)

Orbit average power (W)

Downlink data volume (Mbit/day)

Size(L x W x H)
(cm)

Active cooling

CIPS

24

35

1000

73 x 40 x78

CDE

2

5

< 1

50 x 34 x 5

SOFIE

38

40

200

58 x 44 x 70

16 TECs, 208-260K

Spacecraft bus

133

163

400

309 x 154 x 133

Total

197

243

1600.6

Table 1: Overview of instrument mass, power, data rate and dimensions

The instrument mass values (Table 1) include the electronic boxes. The bus size includes the solar arrays in deployed configuration.

CIPS (Cloud Imaging and Particle Size):

CIPS is an instrument designed and
developed at CU/LASP (University of Colorado / Laboratory for
Atmospheric and Space Physics), Boulder, CO. The objective of CIPS is
to take imagery of the clouds to determine when and where they form,
and to document what they look like. 40)41)

Figure 20: Illustration of CIPS (image credit: CAS/HU)

CIPS images the PMC
cloud deck with a resolution of 2 km, and measures the scattering phase
function of PMCs along with other microphysical properties such as
particle size and water content. The instrument consists of four wide
angle cameras with a combined FOV (Field of View) of 80º x
120º. The camera FOV is centered about nadir providing an image
size of 1440 km x 960 km at an altitude of 83 km. The clouds are imaged
at the UV bandpass of 265 nm (±5 nm), taking advantage of the
strong absorption characteristic of ozone at this wavelength to enhance
the contrast of the cloud scattering with respect to the background
Rayleigh scattering. 42)43)

Each camera has an overlapping FOV
and a pixel size at the nadir of ~2 km. The FOV of the camera system is
80º-120º, centered at the sub-satellite point, with the
120º axis along the orbit track as shown in Figure 21.

CIPS is a panoramic UV (narrow
bandwidth with a center at 265 nm) nadir-pointing imager. Each of the
cameras has a custom-designed 9 element lens system (Lakin Optical
Systems) and a narrow bandpass optical filter from Barr Associates. The
UV cloud image is focused onto the CsTe photocathode on a Hamamatsu
image intensifier (converting UV to visible) that is fiber coupled to
an Atmel CCD. An instrument microprocessor stores and processes all
camera images for transmission to the spacecraft (Ref. 34).

The combination of images from the
four cameras is referred to as a scene. CIPS records scenes of
atmospheric and cloud radiance in the summer hemisphere from the
terminator to ~40º latitude along the sunlit portion of the orbit.
The near-polar orbit and cross-track FOV will cause the observation
swaths to overlap at latitudes higher than about 70º, so that
nearly the entire polar cap will be mapped daily by the 15-orbit per
day coverage. In the nominal pointing mode, the CIPS images extend
poleward to about 85º latitude in each hemisphere (Ref. 35).

Each camera has a focal ratio of
1.12, a focal length of 28 mm, a 25 mm lens diameter and includes an
interference filter and a CCD (Charge Coupled Device) detector system.
The throughput of the optical elements and their sizes are designed for
a 71% measurement precision of the background sunlit Earth. The custom
UV filters were manufactured by Barr associates and centered at 265 nm.
The CCD detectors are coupled with Hamamatsu V5181U-03 image
intensifiers (40 mm diameter active area) and have 2048 x 2048 useful
pixels that are electronically binned in 4 x 8 combinations for an
effective 340 (cross track) x 170 (along track) pixel images. The
signal in each pixel is digitized to12 bit resolution. On average, 26
images are produced per orbit in the summer polar region with special
‘first light’ images just beyond the terminator.

Imaging is achieved with this
body-fixed camera assembly using an exposure time of 1 s, which, when
combined with the FOV, yields the nadir spatial resolution of ~2 km.
Between four and seven exposures of the same cloud volume are made
during a satellite overpass, at a rate of one scene every 46 s. Each
CCD is equipped with a DSP (Digital Signal Processing) interface that
incorporates a lossless Huffman compression algorithm, reducing data
volume by about a factor of two. Therefore ,each scene produces 523kB
of data yielding approximately 18 MB per orbit.

Figure 22: Schematic view of a single CIPS camera and its elements (image credit: LASP)

CDE is an in-situ dust detector
designed and developed at LASP. CDE is mounted on the zenith side of
the spacecraft, providing a very wide field of view and looking away
from the Earth. The objective is to measure the influx of dust
particles into the upper atmosphere, the PMC (Polar Mesospheric Cloud)
region.

CDE is a copy of
the SDC (Student Dust Counter) developed for the New Horizons
spacecraft of NASA (launch Jan. 19, 2006), that is now traveling to
Pluto and the Kuiper Belt (Pluto flyby in 2015).

Both CDE and SDC comprise an array
of impact detectors made from polyvinylidene fluoride (PVDF). PVDF is
an electrically polarizable material. When physically impacted by a
high speed particle, a small change in the polarization takes place,
and that depolarization signal can be sensed as a change in electrical
charge by fast analog electronics. Both particle mass and velocity
contribute to the signal.

To minimize redesign in the CDE
effort, the CIPS instrument electronics provide the interface to the
CDE instrument, and the CIPS electronics were designed to mimic the New
Horizons spacecraft interface. The AIM payload originally included an
instrument platform assembly (IPA) as an integrating structure that
would have permitted the instruments to be assembled and tested as a
suite, and then installed on the spacecraft as a complete unit.

Figure 24: Illustration of the CDE device (image credit: CAS/HU)

CDE observations integrated over
several days are expected to show the temporal variability of the
cosmic dust influx that could influence the formation of PMCs. The
cosmic dust delivered to the mesosphere is most likely ablated to
particle radii of ~0.2 nm, which coagulate to PMC nucleation sites of
~1 nm. Recent results from global-scale models reveal that variations
in the influx of meteoric material can dramatically affect the
availability of nucleation sites in the polar summer mesosphere.
Although the availability of nucleation sites depends strongly on
equator-ward transport from the polar summer mesosphere, large
uncertainties exist in the models regarding total influx of material,
the initial radius of the dust and the coagulation efficiency. The
temporal variability of meteoric material measured by CDE will be used
with observed variations in PMCs and model studies to assess the role
that extraterrestrial forcing plays in PMC formation and variability.
Such studies will involve sorting out other sources of variability of
ice properties, and thus will probably require several PMC seasons to
build up an adequate database.

Figure 25: CDE sensors mounted on the top of the spacecraft (image credit: LASP)

Legend to Figure 25: Each patch of the 12 active PVDF sensors has a surface area of about 85 cm2.
The panel is mounted to point towards the local zenith direction at all
times, minimizing the impact rates from orbital debris.

The CDE goal to measure an expected impact rate of ≥ 100 hits/week requires a mass threshold of ≤ 4 x 10-12 g and a total sensitive surface area of ≥ 0.1 m2. To meet this requirement CDE (Figure 25) consists of 12 active PVDF patches with surface areas of 85 cm2
each. In addition,there are two other sensors (identical to the front
side patches), on the back side of CDE that cannot be hit by dust.
These reference detectors are being used to measure the noise
background. The CDE dynamic range provides mass resolutions within a
factor of ≤ 3 in the mass range of 4 x10-12 g ≤ m ≤ 4 x 10-9 g covering an approximate size range in particle radius of 0.8 µm ≤ a ≤ 8 µm.

Each of the 14 sensors has an
adjustable threshold to optimize CDE operations. To follow the possible
degradation of its performance due to ageing, CDE has onboard
calibration capabilities for its electronics. Internal signals can be
injected in each of the 14 channels with amplitudes covering its entire
dynamical range. Each dust hit generates a science event, where the
time, channel number and the impact charge are recorded, in addition to
all relevant housekeeping data. Using appropriate averages this can be
turned in to a time-dependent global map to show the possible spatial
and temporal variability of the amount of cosmic dust entering the
atmosphere.

SOFIE (Solar Occultation For Ice Experiment):

The SOFIE instrument is designed and
developed at USU/SDL (Utah State University / Space Dynamics
Laboratory) at Logan, UT. SOFIE is of SABER (Sounding of the Atmosphere
using Broadband Emission Radiometry) heritage flown on TIMED (launch
Dec. 7, 2001). The objective of SOFIE is to observe the following
atmospheric constituents by the use of the solar occultation technique:
temperature, PMCs, carbon dioxide (CO2), methane (CH4), nitric oxide (NO), ozone (O3) and aerosols.

SOFIE is an
8-channel differential absorption radiometer covering the spectral
range from 290 nm (UV) to 5.26 µm (MWIR). Six channels are
designed to measure gaseous signals, and two are dedicated to particle
measurements. Measurements in two carbon dioxide bands are being used
to simultaneously retrieve profiles of temperature and the carbon
dioxide mixing ratio. 44)45)46)47)

Each SOFIE channel uses two
detectors, one that samples a spectral region where the target gas is
strongly absorbing, and one that samples a weakly absorbing region.
Measuring the difference of these signals allows precise isolation of
the target gas signal. Once the gaseous contribution is isolated, the
remaining signals can be used to infer particle extinction, so that
particle measurements will be obtained from every channel.

Radiation entering the SOFIE
telescope passes through a field stop which defines the instantaneous
field of view (FOV). The field stop provides an angular field of view
of 1.8 arcmin vertical by 6.0 arcmin horizontal. The FOV dimensions at
the tangent point are about 1.2 km vertical by 4.1 km horizontal.

Optics: SOFIE uses a cassegrain
telescope with a 10.16 cm entrance pupil. An elliptical steering mirror
(16.76 cm x 11.55 cm) directs the incoming beam onto a focusing mirror
and then to a secondary mirror (Figure 26).
The backside of the secondary mirror contains a pickoff mirror that
directs a portion of the beam into the sun sensor module. The main beam
passes through a field stop that determines the instantaneous field of
view (IFOV). The field stop is 1.95 arcmin vertical x 4.74 arcmin
horizontal, which is 1.50 km x 3.63 km when projected to the 83 km limb
path tangent point. The beam is chopped at 1000 Hz using a tuning fork
device, and directed into the CSM (Channel Separation Module) where the
science measurements are accomplished.

The SOFIE instrument includes a
solar tracking system with the ability to acquire the sun, track it
through an occultation, and perform scans as required for various
on-orbit calibration sequences. The pointing system consists of two
principal components: the sun sensor and steering mirror.

• The sun sensor uses a
radiation hardened focal plane array (FPA) image sensor with 1024 x
1024 pixels. The FPA field of view is 2.04º in azimuth and
2.025º in elevation. The FPA diodes are 15 µm in size and
subtend roughly 7.14 arcsec at tangent. The sun sensor center
wavelength is 705 nm with a bandwidth of ± 5 nm. Incoming light
is dispersed by the sun sensor optics according to the Airy disc
function.
The sun sensor performs two principal functions, 1) location of the sun
and 2) directing or maintaining the boresight (FOV) at a desired
location on the solar image.

• Pointing is accomplished
using a steering mirror at the aperture entrance. The steering mirror
provides ± 1.6º of rotation in both elevation and azimuth.
Optical gain magnifies these angles by a factor of 2 in elevation and
sqrt(2) in azimuth. As a result, the range of optical rotation provided
by the mirror is 4.5º in azimuth by 6.4º in elevation. The
steering mirror has a maximum slew rate of > 0.8º/s. The
pointing resolution is better than 0.8 arcsec.

Signal conditioning electronics:
Three measurements are accomplished for each channel, the weak and
strong band radiometer signals (Vw and Vs) and
the difference of these signals (ΔV). Output signals from the
detector preamp undergo signal conditioning including synchronous
rectification at 1000 Hz. SOFIE signals are digitized using a 14 bit
converter operating in the range of ±3 V.

SOFIE measurement geometry:
SOFIE provides spacecraft sunset measurements at latitudes between
about65º and 85ºS and sunrise measurements at latitudes
between about 65ºN and 85ºN. SOFIE observes 15 sunrise and
15sunset occultations per day,and consecutive sunrises or sunsets are
separated by ~96 min in time or ~24º in longitude. The SOFIE FOV
(Field of View)at the tangent point is ~1.5 km vertical by ~4.4 km
horizontal. The SOFIE measurement suite,consisting of 16 radiometer and
8 difference signal measurements, is sampled at 20 Hz, which
corresponds to a vertical distance of ~145 m in the atmosphere. The
vertical resolution of 1.5 km combined with the ~3 km s-1
solar sink or rise rate sets the natural frequency of the data set at
~2 Hz,which is the rate at which the FOV vertical dimension is swept
through the atmosphere.

SOFIE instrument performance: SOFIE
performance was characterized in laboratory calibration studies before
launch and detailed characterizations have been completed in orbit.
Laboratory calibration sequences addressed important instrument
characteristics including measurement background and noise, FOV,
response linearity, relative spectral response,time response,absolute
gain,and difference signal gain. Because the basic measurements are
ratios of signals used to determine atmospheric transmissions, absolute
radiometric calibration is not important,except to ensure that the
exoatmospheric solar view generates signals near the upper limit of the
data acquisition system. Performance of the SOFIE measurement and
retrieval system in-orbit is excellent in all cases with noise levels
at or below laboratory values. The retrieval precision and altitude
range based on data analysis thus far are summarized in Table 3.

The information compiled and edited in this article was provided byHerbert
J. Kramer from his documentation of: ”Observation of the Earth
and Its Environment: Survey of Missions and Sensors” (Springer
Verlag) as well as many other sources after the publication of the 4th
edition in 2002. - Comments and corrections to this article are always
welcome for further updates(herb.kramer@gmx.net).